Field of the Invention
The present invention relates to an image capturing apparatus and a method for controlling the same, and, in particular, relates to an exposure control technique.
Description of the Related Art
There is known to be a method for suppressing blown-out highlights (overexposure) and blocked-up shadows (underexposure) and expanding the dynamic range by raising the luminance of a video image shot with an exposure that is lower than the standard exposure (underexposure) by performing gamma correction (tone correction). Gamma correction characteristics are changed in accordance with the dynamic range that is set, such that the relationship between the level of incident light and the level of output signals becomes a relationship that was prepared in advance.
Some methods for expanding the dynamic range and changing gamma correction characteristics have been suggested. Japanese Patent Laid-Open No. 2004-120511 suggests a method for effectively using the tone of a high luminance portion as well by expanding the dynamic range and smoothing the inclination of the tone characteristics of the high luminance portion. According to this method, even in a state where the dynamic range is low, the tone of the high luminance portion can be effectively used by changing the inclination of the tone characteristics of the high luminance portion.
In addition, Japanese Patent Laid-Open No. 2006-81037 suggests a method for shifting the dynamic range in accordance with the maximum reflectance of the incident light amount of an object, and changing gamma correction characteristics so as to correspond to the maximum reflectance, while maintaining output with respect to a reference incident light amount. In this method, in the case where the maximum reflectance is high, the gamma value is brought close to 1, and in the case where the maximum reflectance is low, the gamma value is brought close to 0. Accordingly, if the maximum reflectance is high, the contrast is reduced, and if the maximum reflectance is low, the contrast is increased.
Incidentally, in the case of expanding the dynamic range, the relationship between the level of incident light and the level of output signals is maintained by shooting with underexposure and changing gamma characteristics. This is substantially the same as signal amplification, and causes decrease in the S/N ratio. Therefore, from the viewpoint of the S/N ratio, it is preferred not to perform dynamic range expansion more than necessary. Therefore, in the case where blown-out highlights occur in an image under a certain shooting condition, it is thought that a user will desire the least amount of dynamic range expansion that can resolve the blown-out highlights. However, in the portion with blown-out highlights, the photometric values are also saturated, and it is not possible to specify the level of the incident light in the portion with blown-out highlights. Therefore, there is a problem in that the user cannot figure out a dynamic range expansion amount necessary to avoid the blown-out highlights.